1. Field of the Invention
The principal utility of the invention is to improve the efficiency of motor vehicles and thus reduce green house gas emissions. More specifically, the field of the invention is hybrid vehicular drivetrains combining at least one hydraulic motor with an internal combustion engine.
2. Prior Art
Hydraulic hybrid vehicles utilize accumulators to store mechanical energy which is recovered from braking the vehicle and/or excess energy generated by the engine. See U.S. Pat. No. 5,495,912 and U.S. patent application Ser. No. 09/479,844 (pending) for details of the use of accumulators in hydraulic hybrid vehicles. However, control of the flow of high pressure hydraulic fluid into and out of the accumulator represents a potential safety problem in the use of hydraulic hybrid drivetrains.
Conventional accumulators are made in several designs including: piston accumulators wherein the piston in a cylindrical accumulator vessel separates hydraulic fluid from a gas (usually nitrogen) which is compressed to store energy by liquid flowing into the vessel, bladder accumulators which use an elastic bladder to separate the hydraulic fluid from the gas, and diaphragm accumulators which use a diaphragm to separate the hydraulic fluid from the gas.
FIG. 1 shows a cross section of the liquid entrance and valve end of a conventional bladder accumulator 10 which is a cylindrical vessel with domed ends. Pressures up to 5,000 pounds per square inch (psi) are common for such a high pressure accumulator that would be used on a hydraulic hybrid vehicle. Hydraulic fluid is pumped into and discharged out of the accumulator through port 11. The liquid flows around poppet valve 12 into the liquid chamber 13 of the accumulator. The accumulator walls 14 must be sufficiently strong to safely contain the high pressure liquid. A compressed gas (usually nitrogen) is contained within a sealed, elastic bladder 15. Spring 16 keeps valve 12 open for normal operation. Valve assembly 17 can be removed from the accumulator if necessary. For a 5,000 psi accumulator, the gas in bladder 15 is usually pre-compressed to between 1,600 and 2,000 psi before any liquid is pumped into the accumulator, to maximize the energy which can be stored within the accumulator. When the bladder 15 is pressurized by admitting high pressure gas through a valve in the other end (not shown), the elastic bladder 15 expands against poppet valve 12 and compresses spring 16 to shut valve 12. With valve 12 shut, bladder 15 is prevented from being extruded through fluid port 11 and rupturing the bladder. Hence the name commonly given to valve 12 is xe2x80x9canti-extrusion valvexe2x80x9d, as this is its design function. When liquid is then pumped through port 11 at a pressure higher than the bladder pre-charge pressure, valve 12 is forced open and liquid flows into chamber 12 compressing bladder 15 and the gas contained therein. When sufficient liquid is pumped into chamber 13 to compress the gas in bladder 15 to 5,000 psi, the volume of the gas and bladder is reduced to approximately one third of its original volume, and substantial energy is stored in the compressed gas. When power is needed by the driver of the vehicle, liquid may be allowed to flow from the accumulator to a hydraulic motor to propel the vehicle. As liquid exits the accumulator, the bladder 15 expands. If liquid continues to be withdrawn down to the bladder 15 pre-charge pressure, the bladder will push against valve 12, shutting valve 12, stopping the further withdrawal of liquid and preventing extrusion of the bladder 15. Spring 16 prevents the flow of liquid out of the accumulator from pre-maturely shutting valve 12.
Anti-extrusion valve assembly 17 performs well in conventional applications of hydraulic accumulators. However, additional valve functions are necessary for the utilization of an accumulator in a hydraulic hybrid vehicle. In the prior art these additional valve functions can be provided only by utilizing separate valve assembles.
Accordingly, it is an object of the present invention to improve safety of hydraulic hybrid drivetrains by improving control of high pressure hydraulic fluid into and out of an accumulator in the drivetrain by provision of an improved accumulator shut-off valve.
The present invention provides a unique means for providing the function of preventing the extrusion of the bladder when the liquid content approaches zero and the pre-charged gas in the bladder (at 2000 psi for a 5000 psi accumulator, for example) would otherwise force the bladder out of the accumulator.
More specifically, the present invention provides a shut-off valve for a hydraulic accumulator in a hybrid vehicular drive train which includes a valve body having a cylindrical hollow with a valve seat surrounding one end of the cylindrical hollow and, slidably mounted therein, a piston including a piston head having a central opening for receiving the stem of a poppet valve having a head which mates with the valve seat in a closed position. The central opening in the piston head which receives the valve stem serves to guide axial movement of the poppet valve relative to the piston. A spring is mounted between the head of the poppet valve and the piston head so as to urge the valve head away from the piston to an open position. The piston operating means, e.g., a control valve, serves to move the piston relative to the valve body between open and closed positions.
In the preferred embodiments, the spring between the head of the poppet valve and the piston head has a strength providing compression force equal to a pressure drop across the valve at a predetermined maximum flow rate, whereby the valve is closed by a flow rate exceeding the predetermined maximum flow rate, thus providing the so-called xe2x80x9cflow fusexe2x80x9d feature of the present invention.
In one preferred embodiment, the piston and the valve body have defined therebetween an annular chamber wherein pressure is controlled by the piston operating means. The piston has at least one flange extending into and dividing the annular chamber and sealing against the inner wall of the valve body. The piston flange divides the annular chamber into a second chamber which is in constant communication with the low pressure reservoir and a first chamber which is in communication with the piston control means, e.g., control valve, for switching pressure in the first chamber between a high pressure source for moving the piston to an open position and a low pressure reservoir for allowing the piston to move to its closed position. Preferably, the control valve is a normally closed valve with the poppet valve being closed when the control valve is in its normally closed position. In one preferred embodiment, the piston has two flanges extending into the annular chamber to define first, second and third chambers wherein the third chamber is constantly open to the cylindrical hollow of the piston.
It is further preferred that the shut-off valve of the present invention be provided with at least one sensor for determining flow rate through the hollow interior of the piston (xe2x80x9ccylindrical hollowxe2x80x9d). Flow rate can be determined by use of two or more pressure sensors spaced along the flow path for the purpose of measuring pressure drop which can be used to calculate flow rate. An electronic control unit or computer receives signal from the sensor(s), computes the actual flow rate based on the signals and compares the actual flow rate against the commanded flow rate. If the actual flow rate exceeds the commanded flow rate, the electronic control unit issues a command signal to the control valve to close the poppet valve.
In a preferred embodiment the present invention also provides a new feature referred to herein as a xe2x80x9cflow fuse.xe2x80x9d If the accumulator outlet line is ever broken or mistakenly opened and the flow exceeds a pre-determined level that would otherwise be the maximum intended flow rate, the valve automatically shuts off. In this preferred embodiment the spring holding the valve open is calibrated so that it allows the valve to close whenever the flow exceeds the pre-determined maximum allowable rate.
The present invention also provides for more rapid closing of the valve and opening of the valve. Very rapid closing of the valve (generally less than 50 milliseconds) is provided in response to an electronic command. The valve may be commanded to close if the computer senses that the outlet flow rate is higher than that expected for that instant, suggesting a leak in the system smaller than that which would trigger the emergency xe2x80x9cflow fusexe2x80x9d shut off. The computer controls and therefore continually knows the outlet flow rate and by comparing the pressures at two locations in the outlet line (to determine a pressure drop which can be correlated to flow rate), or by other flow rate measurement means, and continuously compares the commanded flow rate to the measured flow rate. If the measured flow rate exceeds the commanded flow rate, the computer will command the valve to shut. The computer also commands the valve to shut when the system is turned off, e.g., when a key is turned off. The command to close (or shut) results in a very rapid closing since the pressure is essentially equal on both sides of the valve when it is open, and the closing force must only overcome valve friction and provide the desired acceleration.
Opening the valve after a period of more than a few minutes (when the pressure downstream of the valve has dropped) requires a very large actuation force because it must not only overcome friction and accelerate the mass of the valve assembly, it must also overcome the force of the pressure in the accumulator acting on the accumulator side of the valve. In the extreme, when the downstream pressure reaches its lowest value (for example, 100 psi) and the accumulator pressure is at its highest value (for example, 5000 psi) a very large force is required to open the valve. For example, if the poppet valve face area is one square inch and the pressure difference is 4,900 psi, then the actuator would have to overcome an additional 4,900 pounds of force to move the valve. Opening the valve of the present invention requires a much smaller actuation force since a small parallel line connects the accumulator to the downstream side of the main valve and contains a small on/off valve which is first commanded to open to equalize the pressure downstream of the main valve with the pressure in the accumulator. The only flow in the small parallel line is that required to pressurize the downstream volume, which is very small. When the pressure downstream of the main valve is the same as the accumulator pressure, the actuation force need be only just sufficient to overcome friction and to accelerate the mass of the valve assembly at the desired rate.
The present invention utilizes a captive o-ring (or similar sealing material) in the poppet valve seat to provide for positive sealing with zero leakage. This prevents the accumulator from slowly losing pressure due to the seal slowly leaking as it would absent a positive seal.